Lithium secondary batteries have received attention as high-capacity power sources for portable and other appliances. Further, lithium secondary batteries have recently been receiving attention as high-output power sources for electric vehicles and the like. Chemical batteries such as lithium secondary batteries usually have a separator that electrically insulates a positive electrode from a negative electrode and holds an electrolyte. In the case of a lithium secondary battery, a micro-porous film made of polyolefin (e.g., polyethylene, polypropylene, etc.) is mainly used as the separator. The electrode assembly of a lithium secondary battery is produced by winding the positive electrode and the negative electrode, with the separator interposed between the two electrodes, into a cylindrical shape or a substantially elliptically cylindrical shape.
Cylindrical lithium secondary batteries are used, for example, as power sources for power tools and notebook PCs. A cylindrical lithium secondary battery is sealed by crimping the opening edge of its battery can onto the sealing plate. In order to fix the sealing plate around the open top of the battery can, the battery can has a step (narrowed part) with a reduced inner diameter in an upper part of the side wall thereof. Japanese Laid-Open Patent Publication No. Hei 11-354084 proposes a high capacity design in which the width B (38 mm) of the negative electrode and the distance A (39.7 mm) from the narrowed part to the outer bottom face of the battery can satisfy the relation: B/A=0.957.
Prismatic lithium secondary batteries are used, for example, as power sources for cellular phones and digital still cameras. Since prismatic lithium secondary batteries are more easily accommodated in an appliance than cylindrical ones, they are becoming increasingly popular. In the case of prismatic lithium secondary batteries, the lead connecting an electrode with a terminal easily comes into contact with the battery can, unlike cylindrical ones. If the lead whose polarity is opposite to that of the battery can comes into contact with the battery can, a short-circuit occurs. It is thus common to place an insulator (hereinafter referred to as an upper insulator) between the upper part of the electrode assembly and the lid (insulating plate) of the battery can. In order to further enhance the resistance to short-circuits, Japanese Laid-Open Patent Publication No. 2004-31263 also proposes providing an insulator (hereinafter referred to as a lower insulator) between the lower part of the electrode assembly and the bottom of the battery can.
The electrode assembly of a prismatic lithium secondary battery is usually produced such that the distance A from the lower face of the upper insulator to the inner bottom face of the battery can and the negative electrode width B satisfy the relation: B/A≦0.96. The higher the B/A ratio, the higher the battery capacity can be. However, if the B/A ratio is too high, the electrode assembly is susceptible to distortion, thereby causing a direct contact between the positive electrode and the negative electrode, i.e., a short-circuit. In No. 2004-31263, the B/A ratio is set to as high as 0.97 by providing the lower insulator that serves as a cushion.
Meanwhile, when a lithium secondary battery is stored in an environment at extremely high temperatures for an extended period of time, its separator made of a micro-porous film tends to shrink. If the separator shrinks, then the positive electrode and the negative electrode may physically come into contact with each other to cause an internal short-circuit. In view of the recent tendency of separators becoming thinner with an increase in lithium secondary battery capacity, preventing an internal short-circuit becomes particularly important. Once an internal short-circuit occurs, the short-circuit may expand due to Joule's heat generated by the short-circuit current, thereby resulting in overheating of the battery.
Thus, in the event of an internal short-circuit, in order to suppress such expansion of the short-circuit, Japanese Laid-Open Patent Publication No. Hei 7-220759 proposes forming a porous heat-resistant layer that contains an inorganic filler (solid fine particles) and a binder on an electrode active material layer. Alumina, silica, or the like is used as the inorganic filler. The inorganic filler is filled in the porous heat-resistant layer where the filler particles are bonded to one another with a relatively small amount of a binder. Since the porous heat-resistant layer is resistant to shrinking even at high temperature, it has the function of suppressing the overheating of the battery in the event of an internal short-circuit.
In order to realize a lithium secondary battery with high capacity and excellent resistance to short-circuits, the proposal of No. Hei 11-354084 or No. 2004-31263 can be combined with the proposal of No. Hei 7-220759. This combination significantly reduces internal short-circuits, but causes a significant capacity loss when the battery is subjected to an impact, for example, when dropped.
In view of the above problems, an object of the present invention is to provide a lithium secondary battery that is excellent in resistance to short-circuits, capable of preventing a capacity loss due to dropping, and capable of a high capacity design.